1. Field of the Invention
The present invention relates to a method for forming a gate electrode of a semiconductor device, and more particularly to a method for a gate electrode of a semiconductor device which forms a re-oxidation film of a sufficient thickness, using a low temperature re-oxidation process, in order to prevent an interface phase from being formed between a tungsten nitride film and a doped-silicon film for the gate electrode.
2. Description of the Prior Art
As generally known in the art, the width of gate electrodes is becoming smaller as semiconductor devices become more integrated. As the width of conductors becomes narrower, the resistance of the conductors becomes greater. In the same manner, as the width of gate electrodes becomes narrower, the resistance thereof becomes greater.
Recently, a tungsten silicide WSix is used as the gate electrode. Various attempts to extend the usage of the tungsten silicide have been made.
However, since all devices require a high-speed characteristic, the resistance of the gate electrode should be adjusted to be lower.
Accordingly, as a substitution for the tungsten silicide WSix using for the gate electrode material, manufacturers of such devices have proposed a laminate structure of tungsten film W/tungsten nitride film WN/doped-silicon film. It is expected to be used for most gate electrodes.
In the laminate structure, when only the tungsten film and the doped-silicon film are laminated, a reaction occurs at the interface between the tungsten film and the doped-silicon film to form an undesirable tungsten silicide. In order to prevent the occurrence of the reaction at the interface therebetween, a tungsten nitride film is embedded between the tungsten film and the doped-silicon film as a reaction inhibiting material. A small quantity of silicon nitride film is formed at a high temperature at the interface between the tungsten film for the reaction inhibiting material and the doped-silicon film.
After the gate electrode is formed, conventionally, in order to remove damaged layers and a field concentration in a gate edge, a gate re-oxidation process is performed.
A conventional method for forming a gate electrode in a semiconductor device will now be described with reference to FIG. 1. FIG. 1 is a cross-sectional view showing a gate electrode in which a conventional gate re-oxidation process is performed.
After a gate oxide film 2, a polysilicon film 3 and a hard mask 4 for a gate electrode are sequentially laminated on a silicon substrate 1, and the resulting object is etched to form a gate electrode 5. A gate re-oxidation process for the substrate resulting object is performed to oxidize a sidewall of an exposed gate electrode by a predetermined thickness, so that a re-oxidation film 6 is formed at a sidewall of the gate electrode 5.
Due to the re-oxidation film 6, damaged layers of the gate electrode 5 are removed, and thereafter the field concentration in the gate edge is removed.
When forming the gate re-oxidation process, a dry oxidation process is not performed in the gate electrode 5 on which the tungsten film W/tungsten nitride film WN/doped-silicon film are laminated due to an abnormal oxidation of the tungsten film. Thus, a wet oxidation process to uniform a rate of hydrogen and oxygen is performed at a high temperature higher than 1000° C.
However, when the low temperature gate re-oxidation process is performed, the interface between a tungsten nitride film and a doped-silicon film for a gate electrode is formed, thus lowering a characteristic of the gate electrode. In order to prevent that, when the gate re-oxidation process is performed at a lower temperature, a re-oxidation film of a sufficient thickness is not formed at a sidewall of the gate electrode not to remove damaged layers of the gate electrode and a field concentration in a gate edge.